In a study run by Daniel Schacter, the William R. Kenan Jr. Professor of Psychology, and Karl Szpunar, a postdoctoral fellow in psychology, they found that interspersing short quizzes into online learning course can dramatically increase student retention of material.
Schacter and Szpunar split a twenty-minute lecture into four five-minute segments. Some research subjects were presented with math problems after each segment while others were not. In the end, all subjects were tested on the material of the lecture.
The subjects who quizzed in between segments scored higher than all other groups, even outperforming the group that was allowed to review the material from the lecture.
Based on this finding, Schacter believes that quizzes to keep learners’ minds engaged are the most important component of effective online learning.
More detail on the methodology http://news.harvard.edu/gazette/story/2013/04/online-learning-its-different/